As feature sizes in integrated circuits approach 0.25 .mu.m and below, problems with interconnect RC delay, power consumption and signal cross-talk have become increasingly difficult to resolve. It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems.
One material with a low dielectric constant is nanoporous silica, which, as a consequence of the introduction of air, that has a dielectric constant of 1, into the material via its nanometer-scale pore structure, can be prepared with relatively low dielectric constants ("k"). Nanoporous silica is attractive because it employs similar precursors, including organic-substituted silanes, e.g., tetramethoxysilane ("TMOS") and/or tetraethoxysilane ("TEOS"), as are used for the currently employed spin-on-glasses ("SOG") and chemical vapor disposition ("CVD") silica SiO.sub.2. Nanoporous silica is also attractive because it is possible to control the pore size, and hence the density, material strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous silica offers other advantages including: 1) thermal stability to 900.degree. C., 2) substantially small pore size, i.e., at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit), 3) as noted above, preparation from materials such as silica and TEOS that are widely used in semiconductors, 4) the ability to "tune" the dielectric constant of nanoporous silica over a wide range, and 5) deposition of a nanoporous film can be achieved using tools similar to those employed for conventional SOG processing.
Nanoporous silica films have previously been fabricated by a number of methods. For example, nanoporous silica films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate, e.g., a silicon wafer suitable for producing an integrated circuit, by conventional methods, e,g., including spin deposition, dip-coating, spray deposition, and/or combinations thereof. The substrate optionally has raised lines on its surface and preferably has electronic elements and/or electrical conduction pathways incorporated on or within its surface. The as-spun film is typically catalyzed with an acid or base catalyst and additional water to cause polymerization/gelation ("aging") and to yield sufficient strength so that the film does not shrink significantly during drying.
The internal pore surfaces of previously prepared nanoporous films are formed of silicon atoms which are terminated in a combination of any or all of the following species; silanol (SiOH), siloxane (SiOSi), alkoxy (SiOR), where R is an organic species such as, but not limited to, a methyl, ethyl, isopropyl, or phenyl groups, or an alkylsilane (SiR), where R is as defined previously. When the internal surface of the nanoporous silica is covered with a large percentage of silanols, the internal surface is hydrophilic and may adsorb significant quantities of atmospheric water. Even if the film is outgassed by heating before subsequent processing, the presence of the polar silanols can contribute negatively to the dielectric constant and dielectric loss. Previously employed methods for overcoming this limitation and rendering the internal pore surfaces of nanoporous silica less hydrophilic include reacting the internal surface silanols with surface modifying agents, including, for example, chlorosilanes or disilazanes. These reactions, which may be conducted in either liquid or gas phases, result in a (SiO).sub.4-x SiR.sub.x [wherein x is an integer ranging from 1 to 3] surface which is normally hydrophobic and less polar than the silanol group it replaced.
Unfortunately, previous surface modification methods have capped the polarizable and hydrophilic silanol groups with trimethylsilyl groups and/or other organic and hydrophobic moieties as discussed above, that are too readily oxidized by subsequent processing steps and reagents, leave silanols on the pore surfaces. For example, the fabrication of IC devices typically requires oxidizing plasmas that contact the insulating films with the above-described negative results. Another such oxidizing fabrication process is the chemical vapor deposition ("CVD") of an SiO.sub.2 hard mask film onto the work piece. During the deposition of the hard mask the nanoporous silica film can be oxidized and the desired dielectric constant and other electrical properties are lost.
Another problem with nanoporous silica is its low mechanical strength. The low mass density of these materials leads directly to weak mechanical properties, such as modulus and hardness, relative to non-foamed silica films. Various IC fabrication steps, such as chemical mechanical planarization and deposition of conducting metal films, place significant stresses on a nanoporous silica layer, that might cause mechanical failure such as cracking, to occur within the nanoporous silica film.
The difficulties inherent in providing stronger nanoporous silica dielectric films can be appreciated by considering that, for a given dielectric constant (refractive index or density), the density is fixed, at least for a specific chemical composition. At any fixed density, the strength of the nanoporous silica is maximized by having the greatest fraction of solid within the skeleton of the film, rather than as appended surface groups. Thus, for nanoporous silica, the properties of strength and dielectric constant (proportional to material density) can be balanced, in one aspect, by keeping as much of the film mass in the structural elements, and minimizing nonstructural mass, e.g., on the surfaces of the nanometer scale pores.
Thus, for all of these reasons, there remains a need in the art for methods and compositions for producing nanoporous films suitable for the production of integrated circuits that have all of the above-described desirable properties, while minimizing those previously indicated shortcomings of the art.